AFD master took me 2 tries to clear even on master despite all my isis practice
thing is 7 million hp is a metric fuckton even with double isis madness, so you're going to need to have enough RCV to live for quite a number of turns ( i think it took me around 20ish turns to take it down)
i used Isis, Siren, Berserk, Echidna, Green Dragon, Isis
dual chimeras are instantly killed using berserk's skill to make a bajillion water matches. remember separate matches are stronger than combined ones (for example 2 matches of 3 water orbs is stronger than 1 match of 6 orbs) so after activating berserk try to separate the water orbs so you have as many matches as possible while still getting at least 1 green and 1 red match
make sure to stall enough so you can Echidna before the boss gets a single turn, because he puts up a guard immediately. you can stall a bit on Boss Echidna if need be since she has a chance of doing nothing or just messing with your orbs
during the boss fight, match shit like crazy, focusing mostly on water orbs. If he uses his orb change skill, pop berserk and then try to clear all the light orbs as you combo. Use Siren when necessary.
also i just realized i could have used Uriel to not give a fuck about the light orb change but whatever, there's still legend i guess...